Hi Ruth -
Each Canvas user sets their own notification preferences. You could ask or recommend that your students set their notifications in a certain way, but it is not possible to set them for your students.
In the past, I have used an Announcement with a question at the end that they answered in an Inbox message. I offered a tiny bit of extra credit to those who replied within 4-hours, just to see what the response time/pattern was. It wasn't perfect, but it gave me a clearer understanding of the response time my students may have if something urgent came up during the semester.
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